1. Field of the Invention
The present invention relates to a motor current detection circuit which detects an overloaded state of the motor based on an increase in motor current.
2. Description of the Related Art
For example, a power window apparatus is provided with a motor for raising and lowering a door glass. In a case in which a foreign object is caught between the door glass and a door frame during the time when the door glass is being raised by the power window apparatus, a variation occurs in the load of the motor. An overload of the motor is detected based on an increase in motor current, and measures are taken such as stopping the upward movement of the door glass, or reversing the direction of movement of the door glass.
In an automatic mode, once the contact for raising or the contact for lowering of an auto-switch is turned on, driving of the motor is continued until the door glass is completely closed or is completely opened even after a driver releases the auto-switch. Therefore, it is useful to detect the motor current so as to carry out the above-described measures against overloading.
When the door glass has been closed completely, the motor may be overloaded causing an increase in the current flowing through the motor. Therefore, a full-closure detecting sensor is provided in the proximity of a full-closure position so as to discriminate the catching of a foreign object and the full-closure of the door glass.
However, when motor drive voltage for driving a motor varies, motor current also varies accordingly. Therefore, there is a fear that an overload of the motor is erroneously detected. Generally, motor current varies independent from motor drive voltage, but dependent on the load imposed on the motor. However, when motor drive voltage abruptly varies, motor current temporarily varies in accordance with the variation in the motor drive voltage.
For example, when motor drive voltage increases, current increases as in the case of a rush current by an amount corresponding to the amount of increase of the motor drive voltage. In this case, there is a possibility that the current exceeds a threshold value which is determinant for judging whether the current is abnormal or not. Also, when motor drive voltage decreases, current varies as in the case of a rush current but in a direction opposite to that of the rush current, namely in the direction of decrease, the difference between the current and the threshold value considerably increases. In this case, there is a possibility that the current does not exceed the threshold value even when an abnormal state occurs.